1. Field of the Invention
The present invention is a blank clamping device for a machine for trimming optical lenses.
2. Description of the Prior Art
Spectacle lenses must be held in place relative to the tools of a grinding machine for trimming the lens.
During trimming, the lens must be held rigidly on an axis about which it is turned to form the various radii constituting the required shape. The axis may correspond to the optical center of the lens of the patient or to the geometrical center of the shape, for example, and does not coincide with the center of the blank. In addition to this, the great variety of geometrical shapes of the concave and convex surfaces of the lenses means that it is not possible to hold the blank in place by means of rigid interfaces.
Existing blank clamping devices for machines for trimming optical lenses have two jaws for clamping concentric faces of the blank fitted with bearing members each of which cooperates with one face of the blank, at least one of the jaws including a body and a plate carrying one of the bearing members and articulated to the body by a ball-and-socket joint.
In an arrangement of this kind the perpendicular to the mean tangent of the surface to be clamped is clearly apparent regardless of the inclination of the perpendicular and the arrangement can therefore accept all geometrical lens shapes.
However, the problem arises of transmitting rotational torque through a ball-and-socket joint. Torque can be transmitted only by friction between the male and female members of the joint, which cannot be increased by increasing the clamping force, as this would break or damage the blank, and which degrades the performance of the ball-and-socket joint.
A previous proposal for transmitting a torque in a ball-and-socket joint is such that the surfaces "stick". Apart from the fact that transmission is then not very positive, this proposal has the drawback of requiring manual "unsticking" of the two components of the ball-and-socket joint after each operation, to avoid breaking the lens.
Another proposal associates two orthogonal pins with the two spherical members of the ball-and-socket joint. This arrangement leads to transmission of torque with too great an angular play, which can be measured in degrees.
An object of the invention is to avoid the above drawbacks and to propose a clamping device incorporating a ball-and-socket joint which improves the performance of the trimming machine in terms of speed of machining, fidelity of shapes and compliance with dimensions and with the axis of the lens.